Planographic printing plate



Patented May 16, 1944 PLANOGRAPHIC PRINTING PLATE William B. Wescott, Dover, Masa, assignor to Addressograph-Multigraph Corporation,

Wil-

mington, Del., a corporation of Delaware No Drawing. Application January 4, 1940, Serial No. 312,390

21 Claims. (01. 10\1149.2)

This application is a continuation-in-part of my copending application Serial No. 161,970, filed September 1, 1937, and the invention herein relates to an improvement in planographic print-,

ing plates and more particularly to a printing plate having a printing face composed of substantially pure cellulose, characterized by having a. substantial portion thereof in the form of a non-fibrous unifying continuum; by a lateral water-diffusion rate of not more than 25 millimeters in four hours; by a smoothness of not less than-two; by the substantial absence of motile resinous pitch"; by a maximum content of resinified extractives of less than 0.5 per centum;

andby substantially none of said resinifled extractives being on the printing face; and to a step in the method of making such a printing plate which comprises reducing the lateral diffusion rate in the substance of at least the printing surface of the printing face thereof preferably but not always or necessarily by exposing such surface to an oxidizing influence, which may be and preferably is a gas, whereby the substance is oxidized at least at the surface thereof temporarily altered condition of density which would result, for example. from calendering the sheet after formation. it being well known that calendering may give the sheet increased density, decreaied porosity. and a changed diffusion rate, all of which characteristics may be restored to normal by soaking and drying the sheet.

Planographic printing depends, as did the older lithographic printing. upon the immiscibility of oil and water; and, as in the case of zinc for instance. upon an actual modification of the surface by both the image-forming material and t e dampening etch whereby the area underlying the image is rendered preferentially wettable by fat v acids and the whites or non-printing areas are rendered preferentially water-wettable. as by adsorption thereon of a lyophilic substance such as gum arabic, for instance.

Planographic printing. from cellulose surfaces of the types adapted to withstand the attritional forces inherent in the printing operation and so facsimile reproduction,

continuity of the image surface and thus not only disposed that the swelling of the cellulosic materialconstituting and immediately underlying said surfaces consequent to the dampening thereof is restricted to a direction normal thereto whereby the dimensional integrity of the image is maintained, depends upon the inherent relative sorbtivity of the cellulosic surface for water-immiscible image-forming materials and for water, and not upon an actual modification of the surface by the image former and the etch as in the case of zinc, as above mentioned.

A plate embodying this invention possessesthe advantages that, due to its composition, the tendency of the lateral diffusion of the dampening fluid to displace the printing image is greatly reduced and that, due to the absence of traces of motile "pitc and the small amount of resinified extractives therein, the printing face thereof is substantially free from autogenic ink receptive spots. Moreover the step of oxidizing the substance of the printing face of the plate is preferq ably but not always or necessarily preceded by predisposing the substance to accelerated oxidation, such predisposition'being either before or after the manufacture of the plate. One preferred and convenient way of oxidizing the substance of the printing surface of the plates is by causing the plates to b slightly concave on the printing side and disposing them in face-toface relation under such conditions as do not entirely obliterate said concavity.

The possibility of using a cellulosic surface for planographic printing had been adumbrated by several suggestions prior to my Patent No. 2,134,165, dated October 25, 1938, in which patent a fundamental prerequisite to successful operation is disclosed. These prior suggestions failed to take into account the fact that, in contrast to metals and stone, cellulosic sheet material when dampened with water inevitably swells; and the further fact that the lateral swelling. thereof. which may amount to as much as 12-15 per centum, not only destroys the dimensional integrity of the image thereby preventing exact but also breaks up the de rades orgrays the reproduction but greatly reduces the printing life of the image.

In my said Patent No. 2,134,165 and my Patent No. 2,205,998, dated June 25, 1940, means of preventing the objectionable lateral swelling are set forth together with certain other improvements over prior suggestions. Nearly two years of commercial experience with cellulosic printing plates made according to said patents and an intraces of motile "pitch in cellulose derived from wood pulp for instance, which in the normal state do not take ink but which tend to migrate to the surface of the sheet where in time" they become preferentially ink-wettable so that as the 1 printing sheet is used it becomes more or less badly speckled with spots which are both inkreceptive and retentive and hence print.

It is an object of this invention to provide a method of obviating this highly objectionable feature of cellulose derived from wood pulp and of'providing an accelerated test whereby the ef-' fectiveness of the .method for any given batch of paper may be ascertained. a

It is another object of this invention to provide a cellulosic printing plate substantially free from the tendency to autogenic spotting as demonstrated by a test adapted to acceleratedly develop autogenic spotting in an'otherwise similar cellulosic plate containing little if any more than the commercial minimum of non-cellulosic extractables normally associatedwith highly purified wood pulp.

A still further object is to provide a cellulosic printing plate characterized by the substantial absence of motile resinous pitch and by amaximum content of resinified extractives of less than 0.5 per centum, substantially none of said resinifiad extractives being on the'printing-face.

It was further discovered that spotting could and did result from mere pressure contact between the substance of the; printing face of cellulosic plates and other papers-as the cellulosic backing component of composite cellulosic plates for instance, and more particularly when such papers are sized with rosin.

It is an object of this invention to provide a step in the method of making cellulosic printing plates whereby contamination of the printing face thereof, by contact with the backing component thereof is avoided.

It is another object of this invention to provide a method of packing cellulosic printing plates in face-to-face relation whereby contact between the backing component and the printing face of adjacent plates is avoided.

Other objects and advantages will become parent as the description proceeds.

Another discovery is related to image retention. It is stated in the above-mentioned patents that greasy image-forming materials could be so integrated with the printing surface of a cellulosic plate of the type described as to yield editions of commercially acceptable length. While this is true, it was discovered that different batches of paper differed greatly in the lengths of edition which they'would yield for the same image integration treatment, and that there was also an apparent difference in water-wettability between papers otherwise apparently alike but from diiferent batches. Differences in the amounts of almost spectroscopically small traces of metal salts normally associated with cellulose derived from wood pulp were thought to exist and those papers having the estimated greater amounts of copper salts (believed to be copper sulphate) were apparently less water-wettable than were otherwise like papers with lesser estimated amounts of said salts.

thereof.

ink repellent and hence required much more' delicate adjustment of the dampening mechanism. Moreover, more alert supervision by the operator was required because it was found to be much more difiicult-and indeed sometimes impossible-to clean up suchink-sensitive backgrounds or non-printing areas become contaminated with ink;

It is well known that a sheet of substantially pure unified cellulose is readily wetted by water and within the limits of porosity and smoothness hereinafter set forth, when moistened is'reonce they had pellent of ink, and it is one of the valuable features of substantially pure cellulose as a printing face that, when of sufficient density and-as I have discoveredeven though oxidized to an oxidation-products content ten to fifty times greater than that of an otherwise like commercial product, the whites or non-printing areas will clean up after a few revolutions of the press and even though, through some momentary failure of the dampening mechanism, they have become heavily contaminated with ink.

Sheet cellulosic material unified by a nonfibrous cellulosic continuum, whether said continuum be derived in situ or be added as an impregnant coating to a preformed fibrous web or be entirely devoid of fiber, is not commercially available free from at least spectroscopic traces of non-cellulosic materials such as the oxide or salts of metals and traces of organic matter of a resinous nature which, in the case of cellulose derived from wood pulp, is commonly referred to as "pitch."

Some of the ,m'etal salts, as copper sulphate for instance, are capable of markedly depressing the water-wettability of a unified cellulosic material in which they are present in substantial amount; but, because of the difficulty of controlling the addition to and the uniformity of dissemination through unified cellulosic material of minute amounts of such salts and hence of obviating the objectionable tendency to background toning which results from too great inclusion thereof and because of the consequent necessity of resorting to the usual gumming up operation which is objectionable, it is considered only from the same batch of paper but even from the same roll of paper varied greatly depending .upon the length of elapsed time between the removal of the paper from the roll and its use as a printing plate on the press. And as between batches of paper made at different times but otherwise as nearly alike as it is commercially possible to make them, it was found that, for papers of approximately the same age, and the same elapsed time between removal from the original roll and printing therefrom, the length of edition obtainable under like conditions was substantially the same; but, if papers otherwise alike but taken from rolls of greatly different age-as two weeks and two years, for instance, it was found that a substantial increase in edition length required an elapsed interval, between removal from the roll and printing, of several months for the freshly made paper whereas the older paper required an elapsed time of but a few days. This was all the more surprising in view of the fact that, with all other factors equal and when these papers were used as printing plates within a few hours of removal from the roll, the lengths of edition obtained from the freshly made and from the old paper were substantially the same. I

It was also found that thebeneficial effect of elapsed time was to a great extent dependent upon the manner in which the printing plates, after fabrication and before use, were stored. Thus it was observed that the efficacy of elapsed time was increased by storing the plates on edge-in which case there was little or no pressure contact; and that it was decreased by storingin the normal horizontal way in pileswhich, if the piles are large, involves of necessity a substantial contact pressure which varies with t e height of the pile and the position of individual plates within the pile.

The beneficial effect of storage on edge was found to be enhanced and more uniform when, for other reasons which will hereinafter appear, the plates were made slightly concave on the printing side and were disposed in face-to-fa e relationship. The opposed concavities thus obtained tended to limitcontact of the printing surfaces to the edges thereof and thus facilitated access of air to these surfaces whereby substanial improvment in image retention was acceleratcd.

Investigation disclosed (1) that the same effect could be produced in shorter time by heating the plates for several days at a temperature of circa 150 F.; (2) that both when plates are stored for a long time at ordinary temperatures and for a shorter time at higher temperatures under conditions admitting of relatively free access of air to the paper constituting the printing face thereof, a slight-degradation ofthe paper had taken place, as reflected in an increase in the copper number and the percentage cold alkali solubility thereof; that is to say, that there had been a slight increase in both the oxycellulose and the hydrocellulose content of the paper; (3) that all unified, substantially pure ,cellulosic slieet materials, may be oxidized more or less readily, depending upon the chemical treatment of the cellulose prior to fabrication into sheet form: and that, in general, sheets of unified cellulose derived from sulphite wood pulp are more readily oxidized than otherwise like sheets derived from cotton; (4) that a rapid and striking qualitative test, capable of roughly indicating the reactivity of a given specimen of unified cellulosic sheet material in response to a precisely controllable oxidizing influence, is afforded by the difference in "feathering on normal and oxidized areas of the same specimen: (5) that ultraviolet irradiation, whether artificial or natural, produced in a few minutes an oxidation effect equivalent to thatrequiring several months contact with air at room temperature; (6) that storage of parchment (up to two years. at least) in the customary tight rolls was not effective to produce a sheet having that desirable surface characteristic which is produced in like stock by three or four months storage under such conditions as permit of relatively free access of air to the surfaces of the freshly made stock or of old stock, the surface of which has not been exposed to oxidizing infiuence, is not a satisfactory criterion of pres performance although it has been observed that. in general, the higher the initial oxidation the more rapid is that increment of oxidation obtained on exposure to air, which increment is a prerequisite to improved press performance; (9) that, under conditions admitting of relatively free access of air to the surfaces, an increment of oxidation-products content corresponding to at least a ten per centum increase in the sum of the copper number and percentage cold alkali solubility over that of the stock when taken from the original roll is required, everything else being equal, toflyield an increase in press performance of circa 1000 impressions; (10) that the press performance further improves as surface oxidation increases, to a point where the image may show a slight tendency to fatten as printing progresses. Such fattening has been observed to occur at circa fifteen thousand impressions when taken from a parchment-faced plate which was so highly oxidized as to be markedly discolored and as to have become perceptibly embrittled. The increment of oxidation-products content of the immediately above-mentioned plate, over that content which obtained when plates of the same stock would not, under otherwise lik'e conditions, produce more than a few score passable copies, was of the order of a fifty-fold increase Hence the range of the oxidation-products content increment over that of normal fresh parchment is from a minimum of 10 per centum to an undetermined maximum which is presumably not much greater than that corresponding to a fifty-fold increase.

Known methods of determining the oxycellulcse and the hydrocellulose content of cellulose the increment in these values which obtains when the sheet material is so disposed, as by loose packing on edge for instance, as to permit relatively free diffusion of air between the packed sheets. Since it has been found that, when normal freshly-made parchment is so disposed that such relatively free diffusion is possible, a' ten per centum increase in the oxidation-products content (as the sum of the copper number expressed in grams and the percentage cold alkali solubility value are herein designated) will be accompanied by an increase in press performance of from less than a. hundred copies to at least a thousand good copies, it follows that the determination of the oxidation-products content increment serves the useful purpose of indicating the extent of improvement to be expected as the result of any given oxidation treatment in which the printing surface is exposed to the oxidizing influence.

It is quite possible for the oxidation-products content of normal parchment in roll storage to increase more than ten per centum over a considerable period, two years in one observed instance, but such increase is without benefit so far as image retention is concerned. In this instance it was observed, as in several others in which the parchment had been stored in rolls until it had acquired a distinctly yellowish hue, that, while the image retention was little if any better than that of freshly made parchment, storage under conditions which permitted relatively free access of air to the parchment surfaces for only a few days is sufficient to produce a further increase which will result in satisfactory image retention.

Proof that the observed increase in image retention or increase in length of obtainable edition was in some way related to oxidation of the cellulose at the surface thereof was sought by exposing part of the printing face of a plate comprising recently-made facing stock to the powerful oxidizing influence of bright sunlight in one instance and, in another, to ultraviolet from anartlficial source, and then printing from the so-treated plates. Under other like conditions, the image on the irradiated portion of the plate greatly outlasted that on the normal portion thereof and. since the effect of the induced oxi dation was the only variable, this demonstration seemed conclusively to indicate that the presence of oxidation products of cellulose at least at the surface of cellulosic plates greatly improved the image l fe under planographic printing conditions. Moreover it was observed that the image life increased substantially in proportion to increasing exposures of the surface to oxidizing infiuences and that for a given cellulosic stock a measure of the increment of the oxidation-products content afforded a close index of the resulting increase in image life under printing conditions.

It follows that the determination of the increment in oxidation-products content is a useful measure of the effectiveness of any of the various known methods of accelerating oxidation, as stated; but it was found that the value of the oxidation-products increment as obtained from surface oxidation only, and not the total oxidation-products content, was significant as an index of substantial improvement in image retention. Thus it was observed that the oxidation-products content of a freshly-made sheeted stock might have a high value and yet immediately after 2,348,771 v fabrication show no such image retention or length of image life under printing conditions as would result from a fraction of that value obtained in consequence of oxidation induced at the surface after fabrication of the said stock. This seemed strongly to indicate that, whatever the total oxidation-products content, oxidation at the printing surface was the factor controlling improvement in image retention. Surface oxidation, as obtained by ultraviolet irradiation for instance, increases the wettability of cellulosic surfaces by certain water-immiscible fluids; which is of advantage and is more fully hereinafter described.

But surface oxidation cannot be readily distinguished from total oxidation, and hence a search for some numerically expressible measure of the effect of surface oxidation was instituted.

As between sheets of the same batch of paper stock, several informative differences were observed; but in general they were overshadowed by the effects of porosity and of density variations between sheets from different batches. As is more fully hereinafter described, these differof their water-diffusion rate as to yield that improvement in image life which is one of the present objects, as stated, and normal or otherwise similar cellulosic surfaces which require integration of the image-forming material by long contact or heating to yield more than a few copies.

The rate of lateral diffusion of water in cellulosic sheet materials, which are otherwise suitable for planographic printing by known methods and which are also adapted to improvement in image retention according to this invention, is substantially greater than the maximum lateral diffusion rate at and below which is obtainable that improvement in image retention which it is the object of this invention to provide.

While it is true that, everything else being equal, the lateral diffusion of water in sheet cellulosic material varies with the porosity thereof, it is also true that the effect of variations in porosity, within the limits ofdensity and surface smoothness hereinafter set forth, can be offset substantially if not entirely by reducing the lateral diffusion rate in accordance with this invention to that value at and below which a substantia!v improvement in image retention obtains, which is of advantage.

The maximum lateral diffusion of water at or below which substantial improvement obtains is 25 millimeters in 4 hours when the measurement is'made under the conditions hereinafter described.

The lateral diffusion rate value above mentioned is for paper at the upper limit of porosity, as hereinafter recommended; At the optimum density and at an oxidation-products content increment obtainable by subjecting a printing face derived from sulphite pulp to irradiation with ultraviolet light from a quartz mercury vapor arc lamp (Hanovia type) at a mean distance of 10 inches for 15 minutes, the lateral diffusion rate is between 1 and 10 millimeters in 4 hours, depending upon the original oxidation-products 1 content; and the severalthousand additional copies obtainable at said diffusion rate so obtained are illustrative of the improvement which it is one of the objects of this invention to provide. I

The upper limit of oxidation is indeterminate as far as image retention is concerned; but for practical purposes it is indicated-by objectionable discoloration and brittleness of the stock. An oxidation-products content increment so high as to cause a discernible but not objectionable yellowing of the stock and as to reduce the diffusion rate to about 1 millimeter in 24 hours, not only greatly improves the image retention but does so without obiectionably adverse effect on the ink repellency of the dampened whites or background; and this is of advantage.

Anyone skilled in the art will recognize that 'the dampening operation in planographic and face, is meant that,while there is a. detectable decrease in the ink repellency of a dampened printing surface as the lateral diffusion rate of water in the unified cellulosic substance of said surface decreases by reason of oxidation-products content increase, the improvement in image retention is out'of allproportionto the very slight decrease in ink re'pellency, and. that no more than usual care is required to keep the background clean. When once a highly oxidized unified cellulosic surface hasbeen scummed or ink-contaminated, it requires slightly more time to clean up-under printing conditions-than does an otherwise. like but less oxidized surface.

The difference in clean-up time, as between a surface oxidized to a lateral diffusion rate 0125 millimeters in 4 hours and an otherwise likeporosity when those variations do not exceed the[ limits hereinafter recommendedi- A test-whereby the suitabilityof unified cellulosicsurfaces for the present purposes in respect of the moist ink repellency thereof, is hereinafter described.

It'ls an advantage or the improved plates of this invention'that a badly scummed background may be cleaned up without such degradation of the image as results from like action on prior cellulosic plates of high lateral diffusion rate, where image retention is bellevedto be solely a function of absorptive integration ofthe image-forming material with the substance of the printing surface.

It is an object of this invention to provide a cellulosic printing plate characterized by a unifled cellulosic printing surface having a lower lateral water diffusion rate for water admitted through the surface than that normal tov otherwise like unified cellulosic material.

It is another object to provide a cellulosic printing plate having a unified cellulosic printing surface characterized by a lateral Water diffusion rate, as determined by. the method hereinafter described, of not more than 25 millimeters in 4 hours.

As stated, normal parchment in loose storage I by contactjfusion, thus more or less completely other part thereof.

(as contrasted to the normal tight rolls or piles) requires several months to develop that oxidative change which results in a measurable increase in oxidation-products content. Any treatment which causes an accelerated oxidation without marked discoloration or embrittlement and which is commercially feasible may be used to bring about the desired change. Ex- 'posure to sunlight is effective but obviously, not feasible asF-a commercial procedure. Exposure to artiflcially produced ultraviolet light is very effective. Ultraviolet irradiation at 10 inches from a quartz'mercury vapor arc lamp for from 2 to 15 minutes, depending upon the reactivity of the stock being treated, will suflice. Whereas it is commercially feasible, such irradiation is expensive and tends to unduly discolor the paper. Other known ways of oxidizing cellulose, including the application of non-gaseous oxidizing agentsthereto, have been tried and found to yield more or less satisfactory results.

The preferred method comprises predisposing the paper to accelerated oxidation by contacting said paper with anhydrous chlorine gasfor a time depending upon the reactivity of the stock and the length :of the desired'time interval between such treatment and the time at which the minimum increment of oxidation products is desired. The longer the exposure to chlorine the shorter is the time required to oxidatively condition the paper. Inwpractice it has been found expedient to subject the paper, previously conditioned to a moisture content of circa 5-6 per centum, to chlorine contacts of from 5 to 10 minutes which, in the case of a readily responsive material-41s sulphite pa'tchr'nent, for instance-will provide an accelerated oxidation rate such that a ten per centum increase in oxidation-products content will obtain in from 1 to 3 weeks, depending upon conditions of storage. For less responsive materials, as parchment made from purified but not greatly degraded cotton, a chlorine exposure time of 2 to .6.hours may be required to produce the same effect in the same storage time. If the more readily: responsive material, first abovementioned be exposed to chlorine from 1 to 2 hours, the required minimum increment in oxidation-products "content will obtain in from 1 to 5 1 days, depending as always upon the more or less paper in the normally dry state be used, it is essential in any case that the surface of the stock be accessible to air, oxygen or ozone.

By the expression unifying continuum as herev in used is meant that non-fibrous form of cellulose resulting from gelatinization, by chemical reaction in the case of amyloid parchment and by mechanically accelerated hydration in the case of so-called glassine or greaseproof paper, which continuum not only surrounds the residual fibers as a coating but also serves to unify the whole filling the interstices and forming a non-fibrous continuum from any part of the structur to any Another example of a cellulosic continuum is found in a sheet of regenerated cellulose.

By the expression unified cellulosic material is meant any sheet material composed essentially of substantially pure cellulose, the wet strength of which is primarily dependent upon that of a component non-fibrous continuum which may, as in the case of filmiform regenerated cellulose,

be the sole component of the unified cellulosic Wherever herein the expressions pamaterial. per and parchment are used without qualification, they are to be understood as restricted to unified materials.

By the expression oxidation-products content is meant the total effect of oxidation of the cellulosic component or components of a unified cellulosic material, such as parchment, glassine, greaseproof or regenerated cellulose, as represented by the arbitrary numerical sum of the values obtained for oxycellulose in terms of copper number (expressed in grams) and for hydro-cellulose in terms of cold alkali solubility (expressed in percentage) by the more sensitive of the known methods. The only departure from the procedure prescribed for carrying out the usual methods of determining the copper number and the cold alkali solubility is'dictated by the fact that unified cellulosic materials suitable for the present purposes are highly resistant to disintegration by water; hence the usual macerating step is omitted and the determinations are made on the material in sheet form, so pleated as substantially to avoid surface-to-surface contact. While this no doubt introduces a variable manipulative error, it is believed that this error results in a smaller average deviation from the true mean than would result from any attritional disintegration of the unified cellulosic sheet.

The expression planographic printing is used herein, in the commonly accepted sense--which includes rotary offset printing, to denote that printing process in which the surface of the printing plate is substantially plane and in which the printing image is substantially in the plane of the printing surface; that is to say, the printing image is neither raised appreciably above nor sunk below the plane of the printing image. Planographic printing from images directly impressed on the printing face of cellulosic plates does not depend however, as it does in planegraphic printing from zinc plates for instance, upon a chemical reaction between the material of the printing surface and a fatty acid component of the image-forming material.

The expression cellulosic printing plate is herein used to denote a cellulosic structure, such as is more fully described'in my Patent No. 2,134,165 and my Patent No. 2,205,998, characterized by a printing face of substantially pure unified cellulosic material and an integral backing adapted to restrict the swelling of the said printing face to a direction normal thereto when, under printing conditions, said face is dampened.

The expression surface is used herein in a broadly inclusive three-dimensional sense as commonly used, and'not in the strictly two-dimensional sense as used in scientific reference to molecular surface phenomena, for instance. It is believed that the improvement in printing utility herein described depends less upon superficial oxidation of the cellulosic continuum constituting the printing face than it does upon the occlusion of micropores in that continuum by the products of oxidation thereof, whereby the diffusion rate of water therein is diminished and the displacement therefrom of water-immiscible image-forming materials by aqueous fluids is impeded.

The expression image-forming materials or image materials is used herein to embrace-all water-immiscible ink-retentive substances or combinations of ink-retentive substances-with water-immiscible substances which may not themselves be ink-retentive but serve as a vehicle for ink-retentive substances, and which alone or in combination are suitable for the present purposes and are more fully described in my copending application Serial No. 312,319 filed January 4, 1940. Image materials having an aqueous base, such as colloidal suspensions of resins or gums, as shellac for instance, are satisfactorily retained by unified cellulosic surfaces of high density whether oxidized or not and hence the image life of this type of image material is improved by the oxidation of unified cellulosic surfaces to the extent only that the products of oxidationocclude the micropores thereof and to that extent increase the effective density of the said surfaces.

For the present purpose the lateral diffusion rate of water in unified cellulosic material is defined as the height (expressed in millimeters) to which distilled water rises in a sheet of said material over a period of 4 hours when the edges thereof are sealed against absorption of water in any convenient manner and when the sheet is suspended over a quiescent bath of said water at room temperature under the following further conditions: (1) the lower end of the sheet is maintained below the level of the bath, this being conveniently accomplished by folding a bit of sheeted block tin over the bottom edge; (2) the machine direction of the sheet is to be parallel to the surface of the bath; (3) the portion of the specimen above the bath is surrounded by ured off in millimeters.

Feathering, as hereinabove mentioned, is 'a well-recognized characteristic of normal parchment consisting of substantially pure cellulose, and it is exemplified by an irregular spreading of thin aqueous ink drawn across the surface thereof. If a portion of a sample of parchment, having an opacity and a smoothness both within the limits herein described, be subjected to ultraviolet irradiation as elsewhere herein described, and if an eyedropper containing circa 0.25 cc. of a thin aqueous ink having a viscosityand surface tension substantially equal to that of distilled I on the former will be more regular in appearance, The extent of the differences will depend upon the porosity of the specimen and,-for a given irradiation, upon the oxidative responsiveness of the cellulose of which the specimen is composed.

By the expression ink repellency or moist inkrepellency as herein used is meant the resistance ing surface.

of unified cellulosic surfaces to contamination by ink contacted therewith when said surfaces are 7 saturated with a suitable dampening etch preferably devoid of the usual gum arabic. Inasmuch the usual gumming up operation, it is desirable to ascertain that lowering the water-diffusion has not objectionably lowered the ink repellency, which may be done by the hereinbelow described test. As stated, a low lateral waterdifiusion rate implies a higher relative 'wettability by water-immiscible substances. Unifledmellulosic printing plates characterized by a low lateral water-diffusion rate resulting from oxidation of the substance of at least the printing surface thereof are, when said face is moistened with a suitable gum-free etch, sufliciently ink-repels lent for planographic printing purposes. however a low lateral water-diffusion rate is obtained by the addition of non-cellulosic components, as copper salts for instance, the moist inkrepellency may be objectionably depressed and hence a, low lateral water-diffusion rate is of advantage only when the moist ink-repellency remains sufiiciently high for practical 'pmposes. In order to ascertain whether or'n'ot a unified cellulosic plate'exhibiting a low lateral water-diffusion rate. is satisfactory in respect of its moist inkrepellency, it sufiic'es to thoroughly saturate the printing surface thereof with the etching solution described in my United states Patent No.

2,003,268, dated May 28, 1935, comprising ammonium dihydrogen phosphate, nickel or cobalt nitrate, glycerin and water, to remove any superflcial etch with blotting paper or the like, and to then immediately roll up a portion of the sodampened surface with a usual stiff offset lithographic printing ink. If the surface has been insufficiently freed of etch, it may ref se. ink; and,

, tive of a satisfactorily high moist ink-repellence.

The so-treated printing surface is then submerged in. the said etch and allowed to remain undisturbed thereunder for ten minutes or so,

after which a few passes of the inked brayer over When siderations obtain. Thus the only objection to a highly glossy surface is that the initial dampening, in order to prevent ink contamination of the whites, requires a slightly "different technique than that involved in the simple swabbing operation commonly practised. On the other hand, a coarse surface is unsatisfactory for the reproduction of fine-lined characters or images, as normal handwriting, pen drawing, typewriting, et cetera. A velvety mat smoothness is the optimum surface condition when combined with high density.

smoothness is for the present purposes satis factorily represented by the numerical values, in

seconds, obtained by the use of such instruments as are commonly used for the purpose in the paper industry. For typewriting and crayon work a smoothness of 2 0r upwardsas determined by the Bekk smoothness Tester, is satisfactory; while for penmanship, whether writing or line drawing, the smoothness should for best results be 4 or more. The optimum smoothness lies in the range between 4 and 8 and, while satisfactory results can be obtained at values corresponding to glossy surfaces,"a smoothness in the stated range is preferred.

Prior to the discoveries on which this invention rests, it was observed that longer editions could be obtained from microporous cellulosic surfaces than from a dense, relatively nonporous but otherwise like surface, and that the time required for the integration of the imageforming material and the printing surface to yield commercial impressions was in some measure of proportionality related to the porosity and that in general, the greater the porosity within limits, the less time,was required for the image anchorage or integration. And while this is true of usual lithographic or greasy image-forming materials on unoxidized-or very little oxidizedcellulosic surfaces, in which case the retention of the imagesforming material is apparently in some way related to the penetration of the pores the previously inkup area should free-or substantially free-said ares from ink.v

In order to obtain a clean, crisp reproduction of an impressed image, it has been found that the printing surface must be close-textured. A

glossy surface is usable but is neither required nordesirable. The optimum texture is a mat surface of fairly high density and velvety smoothness.

It is well recognized in planographic printing from metals that fine detail in the image is best retained by a metal plate having a very fine, rather than a coarse, grain imparted toits print- A polished metal surface will not do so well because it will not so readily retain moisture sufficiently to stay clean during printing. In planographic printing from unified cellulosic surfaces, similar though not identical conof these surfaces by the image-forming material,

it has been found as part of this invention that the image-forming materials best adapted to be retained byoxidized surfaces are most tenaciously retained by such surfaces when the porosity thereof is diminishingly small. One decidedly detrimental efiect of high porosity is that it tends to cause spreading of the image incident to the integration treatment prior to printing and that it also results in early loss during printing of that crispness of image characteristic of good reproduction.

The density of the unified cellulosic printing surface, when density is considered as substantially the reciprocal of porosity, is a major factor in the life of the image under printing conditions. The expression density is herein used in reference to the compactness of the sheet and its relative freedom from surface porosity. Density of unified cellulose, as amyloid parchment or greaseproof paper, varies with, and may be satisfactorily expressed in terms of, translucency or opacity. Obviously the opacity of a cellulosic sheet of a given density varies with the thickness thereof; and hence numerical values of opacity I reflect comparative densities only for sheets of the same thickness.

an opacty less than 0.45 because lower opacities than 0.45, at the given thickness, indicate such a high degree of parchmentization as results in too great brittleness for conveninet handling. In glassine the low limit of opacity is more indefinite and is of less importance, as it isalso in nonflbrous sheets of regeneratedcellulose.

As stated, best results are obtained with smooth though neither necessarily nor desirably glossy surfaces. A sheet of unified cellulosic material and more particularly amyloid parchment may be satisfactorily smooth in the dry state and yet so rough in the dampened state as to seriousl decrease the lengthof life of an image impressed upon it. This adverse condition arises when there is such a lack of internal uniformity of structure as to result in local differences in the extent to which the sheet swells when dampened.

ture is of course not obtainable in any sheet composed of fibrous material bonded and unified by a continuum derived from gelatinized cellulose, and indeed is not requisite for the present purpose; but gross speckiness is detrimental and should be avoided. The observed effect of speckiness, in a sheet the swelling, of which is restricted to a direction normal to the surface -as is required for the present purpose; is that dampening causes the sheet to swell irregularly thereby raising the small areas over the high opacity specks above the general level of the surface. When 50 irregu larly swollen, the appearance of the surfac under reflected light of grazing incidence is so unmistakably characteristic that it may well be likened to goose-flesh. Any speckiness less than that which upon swelling, when such swelling is restricted to a direction normal to the surface, causes no readily discernible increase in rough nessat the surface when viewed by light of grazing'incidence, is not objectionable. That portion of the image which is raised by the abnormally high swelling of high opacity specks orspots, is subjected to greater attritional wear than is the rest of the image-which is in the general plane of the normally swollen surface; and hence the image wears away more rapidly over the high spots, and the impression soon becomes uneven and unsightly, which is objectionable.

For commercial duplication by the planographic method at least one thousand clean facsimile copies are required. By this is meant that there should be no gross degradation of the copy at the one-thousandth impression, as compared to the first impression. And it is an advantage of the improved cellulosic .plates of this invention by oifset planographic' printing at commercial The resulting local differences in.

speeds of circa 4000 impressions an hour without that long wait for absorptive integration of the greasy image-forming material and the printing face of the plate which is required bythe cellulosic plates described in my hereinabove identified Patent No. 2,134,165; which is of advantage.

It is obvious, as stated. that unified cellulosic material suitable for the purpose of this invention must be free from non-cellulosic substances which are or may become ink-receptive; hence due care must be exercised to avoid the inclusion of such contaminants and scrupulous cleanliness should be observed throughout all stages of manufacture.

Notwithstanding the exercise of great care to avoid contamination of the printing plates made according to my above-mentioned Patent No.

2,205,998, and after upwards of a hundred thouprinting spots. These spots ranged in size from mere pinpoints to spots as large as, and occasionally many times larger than, the usual period mark. The larger spots were usually, of roundish contour and frequently' printed with less density at the center than at the edges. The smaller spots almost without exception printed a solid color.

Investigation showed. (1') that all the spotting which developed in storage on plates that where lucency. Absolute uniformity of internal strucspots-frequently characterized by hollow centers, but did not diminish the occurrence of the smallersolid spots; (4) that for the same incubation period, that is storage in a moisture-saturated atmosphere at 150 F., plates which were stacked in the normal face-to-back sequence under pressure contact in horizontal piles, de-

veloped a far greater number of pinpoint spots than did like plates arranged in an alternating face-to-face and back-to-back sequence but otherwise similarly stacked; (5) that when the alcohol-ether extractives of the kraft backing and of the parchment facing components of the plate were applied in solution to the printing face and the solvent in each case was allowed to evaporate, no appreciable decrease in ink-repellency of theso-treated areas took place when the plate wa dampened and inked up on the press in the usual manner within a few days of such treatment; but (6) that, when a similarly treated plate was subjected to ultraviolet irradiation and then submitted to the same printing test, the areas contacted with the extractives showed a markedly lowered ink-repellency-sometirnes indeed printing a solid color; (7) that when two pieces of the kraft backing, one not laminated and the other laminated to facing material, were cut to distinguishing shapes and held in the incubator in pressure contact with the printing face of a cellulosic printing plate for one or two days and when, after removal of the pieces of kraft, the entire printing face was then irradiated with,

ultraviolet and the so-treated plate run on the press in the usual manner so that the only variables were those localized by and due to said treatment, the surprising result was that the spots in the area contacted by the laminated kraft were many times greater than those in the area contacted by the unlaminated kraftwhich in turn was distinctly more spotted than were the otherwise like areas of the normal plate surface; (8) that plasticizers, such as butyl phthalate, even when present in'very small amount in thermoplastic cellulosic adhesives used as a waterresistant bonding intervenor between the back of the printing face and the backing of a composite cellulosic printing plate, were capable of migrating and did in time migrate through the said backing and greatly increased the spotting of the printing face of a cellulosic plate held in protracted contact therewith; notwithstanding the fact that this plasticizer is not in and by itself ink-receptive; (9) that chlorination of parchment made from fresh sulphite pulp to predispose the said parchment to accelerated oxidation greatly increased the autogenic spotting over that of otherwise like stock stored for like time under like temperature and humidity conditions; but (10) that chlorination of parchment made from sulphite pulp, the pitch content of which had been resinified, did not increase autogenic spot-' ting over that of otherwise like stock otherwise similarly treated; (11) that cellulosic plates, a

representative specimen of which withstands the ment of autogenic spottiness; (12) that the lower the pitch content of the parchment, the less the spotting of a printing plate faced therewith; and (13) that for the same pitch content, the longer the stock-as pulp or waterleaf-was aged before being parchmentized, the longer the incubation period required to cause autogenic spotting.

From the abovelisted observations and others of a confirming nature, it became clear that autogenic spotting may result from mold growth, from pressure contact with rosin-sized paper, and from residual traces of motile resinous matter associated with cellulose fibers derived from wood and commonly referred to as pitch. Mold growth can be prevented by applying traces of a suitable fungicide to the printing surface in known ways. For the present purposes sodium trichlorophenate in circa 0.5 per centum solution, when used to dampen the backing stock prior to lamination, will after lamination and when the so-dampened backing is pressed against the facing of the next layer in the wind-up roll, transfer thereto enough fungicide to adequately protect the said facing from mold growth under ordinary conditions of storage.

. In order to obtain good thermoplastic adhesion between dense parchment and a kraft backing, it is necessary to use very heavy pressures which tend to drive the adhesive into the backing to an objectionable extent. This objectionably deep penetration of the kraft backing by the adhesive may be minimized by heavily sizing the same. Rosin is, in papermaking, commonly used as a sizing and is a satisfactory one for the backing of composite cellulosic plates except in respect of the contamination of the printing face of said plates when they are brought into pressure contact therewith for protracted periods, or at elevated temperatures for short periods.

Contact between facing and backing in the wind-up roll after lamination could obviously be avoided by the insertion of an interleaf, but this was found to be costly inasmuch as the interleaf could not be reused after it had itself become contaminated. A solution of this phase of the contact problem was found in chilling the laminated web in transit from the rolls of the laminator to the wind-up roll, it having been determined that no detectable contamination occurred in a sixty-minute contact period when'the temperature of the stock was reduced to circa 55 F.; and hence, when the wind-up 'of the chilled stock has proceeded for circa minutes, the'web is cut and the leading end of the web coming from the laminator is attached to anotherwind-up roll together with the trailing end of the previously wound-up web, the webs being so disposed that the printing faces thereof arewound up in contact as lamination is resumed, the first-wound web serving as a face-to-face interleaf for the second; and lamination is carried on at such a rate that the maximum face-to-back contact time for the first-wound web is not more than circa 60 minutes. All subsequent operation on the stock are performed'in such manner that all further face-to-back contact is avoided.

Because of the difference in the coefficient of expansion of the kraft backing and the parchment facing, it is necessary to so moisture-condition the backing stock Just before lamination as to obtain substantially balanced contraction strains between the backing and facing of the laminated web. If the contraction strain of the parchment facing is allowed to remain slightly higher than that of the backing, the web will be slightly concave on the facing side; and this is of advantage.

'I'o avoid contamination by contact with the backing, the printing plates are-except for a short time while chilled and immediately after laminationdisposed in face-to-face relation, as stated, and, when stacked loosely on edge or horizontally in small piles of between fifty and one hundred plates, the aforementioned concavity tends to limit contact between adjacent faces to the edges thereof whereby ready access of air to these surfaces is assured. The final package is conveniently a cardboard box of slightly greater depth than the minimum thickness of the stack of plates for which it is intended. This added depth permits the stack of plates to expand by reason of the aforementioned greater contractive strain in the parchment whereby they tend to curl towards the printing face andimpart a slight concavity thereto; which is of advantage for the reasons stated.

The residual traces of pitch in sulphite stock are believed to be associated with very small remnants of medullary rays. These remnants,

when appropriately strained and viewed microscopically, seem in'general to be more or less pointed at the ends and very short and small as compared to the average sulphite fiber. further believed that these small spindle-shaped remnants of fiber are, during the felting operation on the paper machine wire, oriented at an angle towards the surface and with respect to the general lay of the rest of the fiber. i

As a theory useful in visualizing the probable cause of the surface phenomenon of autogenic spotting, and in view of the known facts that It islLO pitch extracted from freshly prepared pulp made from green wood has a low viscosity whereas pitch extracted from pulp made from seasoned chips or from old pulp has a high viscosity, it is suggested-but not relied upon as a statement of factthat pitch of high motility, that is of low viscosity, flows more or less slowly depending upon the temperature along the fibers of its origin into the unifying continuum which constitutes the surface of the parchment sheet,

- and there undergoes that presumably oxidative change whereby it becomes ink-receptive and -retentive. On ageing, as stated, the pitch undergoes a change characterized by increasing viscosity and a tendency towards the solid stateat least at ordinary temperatures. For want of a better understanding as to how much of this change is attributable to oxidation and how much to polymerization, the transformation is herein referred to as one of resinifioation and the resultant solid or at least highly viscous product is, for the present purpose, referred to as resinified extractives.

The determination of the pitch content of a sheet made from wood pulp may be made by extraction in the usual way; and for the present purposes should not be over 0.5 per centum, and preferably lower. A determination of the resinous extractives does not however show whether these extractives were in the motile or resinified state, and consequently a test for potential autogenic spotting is required.

The pitch content of the printing face of a sulphite-derived plate is for the present purposes deemed to have been immobilized by resinification, induced naturally or otherwise, sufficiently to prevent autogenic spotting when-after in cubation for 4 days at 150 F. and subsequent irradiation of the printing face by ultraviolet light from a quartz mercury vapor arc lamp (Hanovia type) for 15 minutes at a mean distance of inches, and then treated as in the hereinabove described test for moist ink-repellencythe macroscopically discernible pinpoint ink-retentive spots are not on the average more than 0.5 millimeter in diameter and not of greater occurrence than one such spot for each square inch of plate surface.

The complete elimination of all traces of pitch cannot be accomplished, so far as is known, by present pulping procedures, and the solvent extraction thereof-if commercially possiblewould be prohibitively expensive and probably not commercially feasible. If sulphite pulp is to be used for the present purposes, and unless and until it can be obtained substantially free from pitch, it-or the waterleaf made from it must be aged either naturally or acceleratedly by heat or other means, before the fiber is gelatinized to form the unifying continuum which is pre-requisite for the cellulosic printing plates of this invention.

Cotton may be used for the present purposes instead of wood pulp; but cotton is more expensive and is not so readily obtainable commercially in a condition which, from the papermakers standpoint, is comparable to the best grades of sulphite wood pulp. Moreover cotton as commercially available for paper-making is slightly more difficult to work up into asheet which will yield after unification the density required for the present purposes, and when made into unified sheets it is somewhat slower tooxidize. When a unified cellulosic plate is satisfactorily made from cotton free from ink-receptive contaminants, of which traces of tarry material are the most troublesome and of the most common occurrence, and when it has been so oxidized as to reduce the lateral diffusion-rate of water to circa 1 to 5 millimeters in 4 hours, it compares favorably with a sulphite plate of the same lateral diffusion-rate. Such a plate derived from cotton has one advantage over that of an otherwise like sulphite plate, which is that the former tends less to discolor for the same ageing time than does the latter. But this dif- V ference hardly justifies the difference in the present level of cost.

While a plate fabricated from purified cotton may, and should, contain substantially no alcohol-ether extractives, it still may contain normally quite invisible traces of tarry material which traces are ink-receptive and -retentive. To ascertain whether cotton or a plate fabricated therefrom contains traces of said tarry material, it suflices to wet the surface of the same with an aromatic solvent, as toluol for instance; whereupon the locus of the tarry material will be disclosed as more or less large brown stains.

That unified cellulosic surfaces, characterized by a low lateral water-diffusion rate and by a high ink-repellency when moist, have ahigher sorbtive cohesion for water-immiscible imageforming materials than do otherwise like surfaces of high lateral water-diffusion rate, has been qualitatively demonstrated in several ways.

A suitable water-immiscible substance having a viscosity comparable to that of water is found in ortho-nitroanisole; and it has been observed that when this substance, colored with a trace of an oil-soluble dye such as Sudan Red for instance. is disposed as a supernatant layer over distilled water, and when otherwise like strips of normal and oxidized parchment are so suspended therein as to cross the interface, the miniscus at the interface is positive for water against the normal or unoxidized strip and positive for the waterimmiscible material (that is, negative for water) against the oxidized strip.

Another striking demonstration of the increased affinity of oxidized cellulosic surfaces for water-immiscible fluids over that of otherwise like but substantially less oxidized cellulosic surfaces is afiorded by the difference in height to which a highlyfiuid, water-immiscible material will creep on said surfaces. Thus, it has been observed that, when two otherwise like strips of normal and oxidized parchment were suspended in a beaker, over and partially immersed in a bath of ortho-nitroanisole tinted with Sudan Red, the vertical creep of this oily fluid in a given time interval was much greater on the oxidized surface than on the, unoxidized surface. This difference in vertical creep is apparently, and for this particular fluid, proportional tothe difference in lateral water-diffusion rate; and the height of rise corresponds in some measure of inverse proportionality to the lateral water-diffusion rate.

That oil creep on an oxidized cellulosic surface differs in more than degree from water diffusion in the substance of said surface is strongly suggested by the observed fact that a continuous line, drawn with an ordinary pencil across one side of a test strip parallel to and slightly above a bath of orthonitroanisole tinted with Sudan Red as aforesaid, will impede the upward creep of the fluid on that side; while the fluid will rise to a much higher level on the side having no transverse pencilled line. Moreover, the appearance.

by transmitted light, of the area above the oilcreep on the pencilled side and when viewed from that side and below the top of the creep on the other side, is strongly suggestive of the absence of any substantial permeation of the substance of the test strip by the red tinted oil.

In view of the above-recited observations, it is believed that the improvement in image retention of this invention is not only related to and in some measure proportional to a decrease in the lateral water-diffusion rate in otherwise suitable cellulosic surfaces whereby the dampening etch, under printing conditions and While effective to render the non-printing areas ink-repellent. is to a substantial extent deterred from lateral diffusion into and under the image areas, but is also in some measure related to an increase in sorbtive cohesion between the oxidized cellulosic printing surface and the water-immiscible image-forming material.

Iclaim:

such plate being composed of substantially pure cellulose, a substantial portion ofsaid face being in the form of a non-fibrous unifying continuum,

, characterized in that the lateral waterdilfusion rate thereof is not more than 25 millimeters in four hours.

2. A planographic printing plate having a unified cellulosic printing face capable of retaining image forming material impressed thereon, when the plate issubstantially dry, against displacement by dampening solutions. and of repelling lithographic inks from its non-image areas when the plate is thoroughly wet with such solutions, such plate being composed of substantially pure cellulose, a substantial portion of said face being in the form of a non-fibrous unifying continuum, characterized in that the lateral water diffusion rate thereof is between 1 and millimeters in four hours;

3. A planographic printing plate having a unified cellulosic printing face composed of substantially pure cellulose, a substantial portion of said face being in the form of a non-fibrous unifying continuum having a smoothness of not less than two and being so oxidized at its printing surface that the lateral water diffusion rate thereof is not more than 25 millimeters in four hours.

4. A planographic printing plate having a unified cellulosic printing face composed of substantially pure cellulose, a substantial portion of said face beingin the form of a non-fibrous unifying continuum so oxidized at its printing surface that the lateral water diffusion rate thereof is not more. than'25 millimeters in four hours.

5. 'A'planographic printing plate having a uni- .fied cellulosic printing face composed of substantially pure cellulose, a substantial portion of said face being in the form of a non-fibrous unifying continuum so oxidized at its printing surface obtained of at least 10 per centum of the oxidation-products content thereof before being so treated and the image retentivity thereof is substantially increased.

7. A planographic printing plate having a unified cellulosic printing face composed of substantially pure cellulose, a substantial portion of said face being in the form of a non-fibrous unifying continuum so oxidized at its printing surface that an increment is obtained of at least 10 per centum of the oxidation-products content thereof before being so treated and the water diffusion rate of the printing face is not more than 25 millimeters in four hours.

8. A planographic cellulosic printing plate having a unified cellulosic printing face and a nonextensible backing to which said face is united, said plate being capable of retaining image-forming material, directly impressed thereon when the plate is substantially dry, against displacement by dampening solutions, and of repelling lithographic inks from the non-image areas, when the plate is thoroughly wet with such solutions, said printing face being characterized in that it is oxidized at its surface-to such an extent that, from surface oxidation, an increment is obtained of at least 10 per centum of the oxidation-products content of the plate before being 50 treated and theimage retentivity thereof is substantially increased. V

9. A planographic cellulosic printing plate having a unified cellulosic printing face and a nonextensible backing to which said face is united,

, pelling lithographic inks from the non-image areas, when the plateis thoroughly wet with such solutions, said printing face being characterized in that it is oxidized at its surface and that the lateral water diffusion rate of the face is not more than 25 millimeters in four hours whereby the tendency of the dampening solutions to displace the image-forming material thereon is approximately at a minimum.

10.'In the manufacture of a planographic printing plate having a unified cellulosic printing face, the step of oxidizing the surface of such face by exposing it to an oxidizing influence to obtain at least a 10 per centum increment in oxidation-products content over that which it would otherwise possess.

11. In the manufacture .of a planographic printing plate having a unified cellulosic printing face,'the step of oxidizing the surface of such face by exposing it to an oxidizing gas to obtain at least a 10 per centum increment in oxidationproducts content over that which it would otherwise possess.

. 12. In the manufacture of a planographic printing plate having a unified cellulosic printing face, the steps of predisposing the substance of said face to accelerated'oxidation and thereafter exposing the surface thereof to an oxidizing infiuence.

13. In the manufacture of a planographic printing plate having a unified cellulosicprinting face, the steps of predisposing the substance of said face to accelerated oxidation and thereafter exposing the surface of said face to an oxidizing gas, said exposure of the surface being accomplished by causing the plates to be slightly concave at their printing faces and disposing said plates in facc-to-face relation under such conditions that the concavity of the faces is not ontirely obliterated.

14. In the manufacture of a planographic printing plate having a unified cellulosic printing face, the step of exposing the surface of said face to an oxidizing gas, said exposure of the surface being accomplished by causing the plates to be slightly concave at their printing faces and disposing said plates in face-t-face relation under such conditions that the concavity of the faces is not entirely obliterated.

15. In the manufacture of a planographio;

printing plate having a unified cellulosic printing face, the steps of predisposing the substance of said face to accelerated oxidationand thereafter exposing the surface of said face to an oxidizing.

gas until an increment in the oxidation-products content of at least per centum is obtained.

16. In the manufacture of a planographic printing plate having a unified cellulosic printing face and a non-extensible backing therefor, -the steps of predisposing the substance of the printing face to accelerated oxidation, uniting the so extent that the lateral diffusion rate of water in such face is reduced to at most 25 millimeters per hour, and uniting said face to the backing therefor.

18'. Inthe manufacture of planographic printing plates, eachpla'te comprising a unified cellulosic printing face and a non-extensible backing therefor, the steps of predisposing the substance of the printing face-forming material to accelerated oxidation, of uniting such printing face material to the backing material, the printing face side being slightly concave, severing the united materials into plates, and disposing the plates in face-to-face relation under such conditions that such concavity is not entirely obliterated whereby the faces of the plates are exposed to an oxidizing gas.

19. A planographic printing plate having a printing element of unified cellulose containing traces of resinous material, a substantial portion of said printing element being in the form of a non-fibrous unifying continuum, said resinous material being in an immobile state to prevent it from. migrating to the printing face of the sheet and there causing autogenic ink-receptive spotting.

20. A planographic printing plate having a printing element of unified cellulose containing traces of resinous material, a substantial portion of said printing element being in the form of a non-fibrous unifying continuum, substantially none of said resinous material being atthe printing surface of the plate and said resinous material being in an immobile state to prevent it from migrating to the printing face of the sheet and therecausing autogenic ink-receptive spotting.

21; A planographic printing plate having a printing element of parchment of wood origin containing traces of resinous material, a substantial portion of said printing element being in the form of a non-fibrous unifying continuum, said resinous material being in an immobile state to prevent it from migrating to the printing face of the sheet and there causing autogenic ink-receptive spotting.

WILLIAM B. WESCOTT. 

